Hey, Aditya_Kaushik. While giving a full explanation would take very long, I used to wonder the same thing myself and so I will save you a lot of time by just letting you know that Pymc3 does not do time series very well at the moment.
There is no inherent reason for this, other than that A) reparametrizing time-series efficiently is a difficult thing, and I believe Stan has much better time-series functionality purely because they accomplish this specific aspect better, and B) the samplers in use were not built for time-series to begin with (unlike SMC, which Pymc3 recently incorporated, but still needs some more work to handle time series as well as it hypothetically can), with reparametrizing only being a stop-gap to not having a dedicated sampler.
These are some things I picked up over time, but I may be wrong, and I hope other users/developers maybe can lend their opinions on the topic ^^; .
There are other threads around this forum, such as in here, that demonstrate similar findings. I wish you the best of luck!