So this is just my two-cents based on working for a VOIP company for a couple years, but I noticed a few patterns, specifically the industry-wide pressure to compete with Twilio, that seem to make it easier to spam text/calls.
1.) VOIP providers have low-barriers to sign-up (typically just a credit card fraud check + check to see if IP or address is from certain countries where fraud calls are common). The up-side to this is that it makes the user experience more friendly to real customers b/c they don't have to wait around, but the downside is that any spammer semi-familiar with the checks that get done can sign up and start sending spam calls for a day or two before most places get complaints and shut them down. The fact that many voip providers allow users to set their Display Id to anything too without any approval also is also really convenient for spammers (ie, there aren't usually ways to immediately detect when a user fraudently set their Caller Id to the name of a bank or something)
2.) APIs everywhere. Twilio got big in part b/c it made APIs available, but the smaller providers are catching up and also adding APIs for texting and calling services. Spammers can automate their spam and re-use the scripts between different accounts when they get banned, so it makes sending out spam texts easier too.
3.) VOIP companies actually are ok with some degree of spam, as long as the spammers pay their bills. So they don't have a motivation to actually stop all of it. Instead of blocking all spam-ish calls outright, some places will just make a deal so that some callers (identified by having a high rate of unanswered calls + very short length calls) pay a higher rate to compensate for potential reputation damage. This is meant for legal cold-calling I imagine, but in practice, they don't know what is actually being said on those phone calls.